Prostaglandin D2 (PGD2) is a major cyclooxygenase metabolite, and its release has been hypothesized to contribute to the inflammation and increased airway hyperreactivity observed in asthma. PGD2 appears to act in part through a recently cloned GPCR designated CRTH2. CRTH2 mRNA expression pattern is consistent with an important role for this receptor in the etiology of allergic disease. The principal hypothesis of this proposal is that critical PGD2 evoked changes in the immune inflammatory response are mediated, at least in part, by CRTH2-evoked responses. We further propose that some of the effects of PGD2 metabolites including PGJ2 and 15d PGJ2 are mediated via the CRTH2 receptor. To test this hypothesis we propose in Specific Aim 1 to define the pharmacology of the mouse CRTH2 receptor. We will express the CRTH2 receptor in cell culture and determine its ligand binding and signal transduction properties. In Specific Aim 2 we will identify amino acid residues critical for ligand binding and signal transduction by site directed mutagenesis. Using modeling and molecular dynamics simulations as a guide, we will introduce point mutations into the CRTH2 receptor, and express mutant receptors in cell culture systems. In Specific Aim 3 we will explore whether PGD2 modulates inflammatory effects and/or bronchoconstriction in mouse models of asthma via the CRTH2 receptor. We will analyze ovalbumin (OVA) allergic sensitization, a TH2 type immune model, in CRTH2 knockout mice and compare this with DP null animals. We further propose to cross the CRTH2 null mice with DP null mice to test the effect of loss of both PGD2 receptors. Completion of these studies should aid in the elucidation of the physiologic role of PGD2 in cellular migration and infiltration in allergic airway disease and will provide insight as to the role of this receptor in mouse models of allergic inflammation.